In order to achieve good performance during reading and writing on an optical disc, calibration management is undertaken during the reading and writing process. Parameters such as write power, tilt, and focus offset, to mention a few, can be optimized. These parameters are dependent on both the temperature and the actual disc. Temperature variations lead to changes of the wavelength of lasers used and disc variations can cause parameter variations in the radial direction of the disc.
Optical dye discs are very sensitive to above-mentioned changes due to narrow margins for these types of discs.
In order to provide a continuous optimal write power, determining an asymmetry parameter, such as beta, is used. This parameter is chosen as it is linear with respect to the laser write power. This linearity is used to determine the regulation direction of the write power.
The jitter or Bit Error Rate (or Block Error Rate, Bler) is parabolic with respect to write power, which disables using this parameter alone to determine regulation direction of the write power.
Beta will thus be determined and this determining is performed at certain moments in time of parts already written. Typically after having written an amount of electronic data the beta value is determined. This determined value is then compared with a certain beta target reference value and the difference is calculated. The difference is further translated to a write power correction. This type of procedure of determining a beta related value, compensates for variations in radial direction of the disc and is denoted Walking Optimal Power Control (WOPC).
The international patent application WO 03/065357 A2 discloses a device for scanning a record carrier and a method for controlling a power of a radiation source, which method includes detecting a sense signal from a beam via a sensor and controlling the laser power to a desired value in dependence on the sense signal, and comprises correcting the desired value in dependence on a correction signal indicative of local optical properties of the record carrier in dependence on at least one sense signal measured on at least one part of the track near the scanning spot. The sense signals are measured on an empty track and on a written track and correction signals are defined as being dependent on linear combinations of said two sensed signals. These correction signals, dependent on measurements on empty and written tracks, define a correction value for the asymmetry parameter beta.
More generally, when writing electronic data on an optical disc using a Constant Linear Velocity (CLV) procedure, the rotation speed decreases with increasing distance from the disc center, as the data is written on the disc.
For Constant Angular Velocity (CAV) writing the rotational angular speed of the disc remains unchanged as data is written on the disc. Hence the linear (tangential) velocity increases with increasing distance from the center of the disc. For CAV writing the beta target value is therefore dependent on the linear velocity.
When accessing a certain position of the disc, corresponding to a certain overspeed factor, Nx, the beta target value, βtNx can be determined by interpolation between two states having well-known speeds or overspeed factors, Nx_min and Nx_max.
                              β          ⁢                                          ⁢                      t            Nx                          =                              β            ⁢                                                  ⁢                          t                              N                ⁢                                                                  ⁢                x                ⁢                                                                  ⁢                _                ⁢                                                                  ⁢                m                ⁢                                                                  ⁢                i                ⁢                                                                  ⁢                n                                              +                                                    (                                                      Nx                    Ns                                    -                                      Nx                                          m                      ⁢                                                                                          ⁢                      i                      ⁢                                                                                          ⁢                      n                                                                      )                                            (                                                      Nx                                          ma                      ⁢                                                                                          ⁢                      x                                                        -                                      Nx                                          m                      ⁢                                                                                          ⁢                      i                      ⁢                                                                                          ⁢                      n                                                                      )                                      ·                          (                                                β                  ⁢                                                                          ⁢                                      t                                          Nx                      ⁢                                                                                          ⁢                      _                      ⁢                                                                                          ⁢                      m                      ⁢                                                                                          ⁢                      ax                                                                      -                                  β                  ⁢                                                                          ⁢                                      t                                          Nx                      ⁢                                                                                          ⁢                      _                      ⁢                                                                                          ⁢                      m                      ⁢                                                                                          ⁢                      i                      ⁢                                                                                          ⁢                      n                                                                                  )                                                          (        1        )            
The speed at the current position Ns is NxNs, and the beta target value at Nxmax and Nxmin are βtNx—max and βtNx—min, respectively. If for instance, 6x<Nx<8x, then Nxmin=6x and Nxmax=8x.
The beta target value for 6x and 8x may be accurately determined by OPC. By linear interpolation the beta target can be determined at any speed. If however, no OPC can be performed at the outside of the disc, the beta target value can be determined based on beta target values from a table and from OPC at 6x, following equations 2 and 3, for known discs.Δβ=(βt6x)table−(βt6x)OPC  (2)βt8x=(βt8x)table+Δβ  (3)
For discs that are unknown to the disc drive, information can be read from a pre-groove, a so called ADdress In Pre-groove (ADIP), to determine the beta target value at non-PC speeds, compare equations 4 and 5, which are similar to equations 2 and 3, shown above.Δβ=(βt6x)ADIP−(βt6x)OPC  (4)βt8x=(βt8x)ADIP+Δβ  (5)
The calculation of the beta target value as well as reading and determining the beta target values on the disc, result in beta values that do not take into account the variations among the actual discs and the difference between the drives that are used. These beta target values are therefore not very accurate.
For this reason, two different problematic effects can occur. The first is an effect called the post heat effect and this effect increases with increasing write power. This jitter effect is manifested in partly collapsing written marks on the disc as the pits and the lands interact with each other on the disc. This jitter effect therefore increases with increasing speed. Due to this jitter effect the regulation direction based on beta could be wrong, which would increase the jitter and Bler parameters and which would result in an undesired write performance.
The second problem is that non-OPC beta target values for non-OPC speeds are not very accurate. For some writing profiles including CAV, the beta target values for in-between speeds are determined based on estimations. This means that write power determination/regulation will be more difficult and may result in a non-appreciated write performance.
There is thus a need to overcome the issue of shortcuts becoming less relevant.